Maybe I am missing the point here, but if you were using split you wound't have to use any of these regexen. (m/(.+?)=(.+)/ as seen above isn't a nice way of dealing with this..) To abstract your problem: You want to eat up everything until an equal sign, and then the rest of the parameter being the key; If there is no equal sign take the entire argument as a key without value. Here's my solution:

my %d;
foreach (@ARGV)
{
    my ($k, $v) = split (m/=/, $_, 2);
    $d{$k} = (defined $v ? $v : "");
}

#this is for checking only..
foreach (keys %d) { print "$_:" . $d{$_} . "\n"; }
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And it does the job.. If you would rather stick to your own code, you might modify it to liik like this:

my $defaults = {};
%defaults = map {(($_ =~ /([^\=]+)\=(.+)/)?($1 => $2):($_ => ''))} @AR
+GV;
print join(",", keys %defaults);
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Regards,
-octo-